(1) Field of the Invention
The invention is related to the rotary wing aircrafts technical domain. The invention concerns so-called rotors in such rotary wing aircrafts, e.g. main rotors capable of providing hovering/forward advance thrust or tail rotors for anti-torque. In some of these rotors, a bar of composite matrix material is provided for joining of the rotor blades to a rotary hub assembly of the rotor.
(2) Description of Related Art
Especially in hingeless main rotors, such a bar has a flat cross section close to the fixation at the rotary hub assembly, for the flat cross section to serve as a flapping hinge while being mainly loaded by flapping moments as well as by lead lag moments. Both moments and their deflections together with the flat cross section for a low moment of inertia in flapping direction and a large cross section for a high moment of inertia in lead lag direction result in a torsion moment at the bar. The high moment of inertia for the bar in lead lag direction is necessary to get enough cross section area for the centrifugal forces acting on the bar and for a minimum stiffness due to dynamic reasons.
In the prior art, the document U.S. Pat. No. 4,648,800 describes a fiber reinforced composite flexbeam. The flexbeam is a structural member in a rotor connector for a rotary wing aircraft to connect the rotor blade to the rotor hub. In order to achieve torsional stiffness and structural integrity, reinforcing doublers are inserted into end portions. Such doublers are contoured to conform to the desired shape of the connector to which the flexbeam will be attached. Such doublers are formed of a torsionally stiff material such as steel, composite material, graphite reinforced epoxy.
The document EP 2246257 describes a flexbeam having a connecting region to a rotor head, with a first connection point and a second connection point that lie adjacent to one the other along a line perpendicular to a radial extension of the rotor head at the end of the flexbeam facing the rotor head. The first connection point comprises a plurality of fastening attachments located one above the other and spaced upwardly and with a circular recess. The second connection point is provided with more fastening attachments.
The document U.S. Pat. No. 5,690,474 describes a composite flexbeam having a pitch region which includes a core laminate of unidirectional fiberglass material and face laminates of unidirectional graphite material bonded to mating surfaces defined by the core laminate. For addressing the problems associated with delamination or splintering in the inboard transition region, this document departs from teaching from older flexbeams that are incorporating an external composite overwrap or an edge cap.
The document JP 2010143484 describes a method for determining the cross section of a flexible beam while reducing a layer-layer shearing stress. The flexible beam has a hub side connection portion, a blade side connection portion to which a rotor blade is fixed via a member allowing feathering motion and lead and lug motion and a flexible portion ranging between the hub portion and the blade portion. The cross section perpendicular to a feathering axis forms inclined faces at both cross ends.
The document US 2006/0204364 discloses a disbond resistant multi-laminate composite product having a first member; a second member of composite material bonded to the first member; a third member of composite material bonded to at least the second member and being so positioned and arranged such that when the second and third members are stressed at least a portion of the third member will have higher stress than the adjacent second member. The third member preferably has a terminal portion adjacent the second member in which terminal portion the strain energy release rate is higher than in the adjacent second member when the second and third members are stressed. The second and third members are preferably so positioned and arranged such that the third member initiates delamination from the second member before the second member initiates delamination from the first member when the first, second and third members are under stress. A stress level indicator is preferably associated with the third member to measure the stress levels in the third member.
The document U.S. Pat. No. 6,375,426 discloses an edge member for use on a composite flexure having an exposed edge surface. The edge member includes an innermost composite ply attached to the edge surface, an outermost composite ply attached to the innermost composite ply, and at least one interior composite ply disposed between the innermost composite ply and the outermost composite ply.
The document U.S. Pat. No. 5,820,344 discloses a helicopter rotor system including a contoured flexure strap having a simplified geometrical configuration. The flexure strap serves to join the rotary hub assembly with the rotor blades and is formed of fiberglass. The primary load carrying fibers of the flexure strap preferably extend in the same direction as the longitudinal axis of the flexure strap in order to withstand the centrifugal forces acting on the strap. A cloth wrap of composite matrix material surrounds portions of the flexure strap to provide redundancy which prevents the fibers in the flexure strap from delamination when subjected to forces arising during operation of the helicopter.